In a preferred form of the invention described in the related applications, a superconductor is produced by the steps of combining a plurality of layers of metal sheets to form a composite structure. The sheets are preferably pure transition metals--niobium, titanium, zirconium, or vanadium, for example--alternate sheets being formed of different transition metals. The resulting composite structure is mechanically reduced sufficiently so that each transition metal sheet is less than 1000 .ANG. thick. In the course of reduction, the composite is subjected to sufficient temperatures for sufficient times such that the transition metal layers are partially reacted to form a ductile superconducting material between the transition metal layers.
Approximately one half by volume of the transition metal layers remain unreacted. These unreacted layers afford efficient flux pinning within the composite when the layers are reduced to the &lt;1000 .ANG. final size. In the fabrication of ternary (or higher order) alloys like NbTiTa, one or more of the transition metal layers is made relatively thin so as to allow complete diffusion through that region. However, at least one half by volume of one of the constituent transition metal layers remains pure or nearly pure after the reaction so as to provide pinning within the ternary composite when reduced to &lt;1000.ANG. in thickness. In other embodiments, powders and filaments can be used instead of initial layers.
Aside from core materials, the multilayer composites described in the examples of the parent applications are composed solely of pure metals. Such composites are undoubtedly useful, but the efficiency of the superconductor fabrication can be greatly improved if one or more of the constituent layers used in the monofilament billet is an alloy, rather than a pure metal. The alloy, which may or may not be a superconductor, can be layered with pure metal or with another alloy, depending upon the superconducting properties ultimately desired. Diffusion in the course of processing this material creates layers of superconducting alloy, which may be similar to, but is not identical with, the initial alloy.
That alloy layers, rather than pure metal layers, are utilized in the present invention in no way affects the core principles of the invention. These principles can be briefly stated as follows:
1. A composite characterized by alternating layers of two or more metals. PA0 2. Processing the composite at elevated temperatures in order to: PA0 3. Mechanically reducing the composite so that the reacted and unreacted layers are less than 1000 .ANG. in thickness.
a. bond the metal layers, thus facilitating reduction of the composite; and PA1 b. create, by diffusion, layers of superconducting material at the metal-to-metal interfaces of the composite, while also maintaining regions of unreacted metal.
In the parent applications, these principles are only applied to pure metal layers, primarily because pure metals are generally less expensive, and thus more desirable, than alloys. However, for reasons related to fabricability and improved performance, the benefits of alloy layers will sometimes outweigh the added costs. The use of alloy layers in accordance with the invention is the focus of the present application.
U.S. Pat. No. 4,803,310, held by Intermagnetics General Corporation, describes a composite characterized by a layered structure of a superconducting alloy and a non-superconducting--i.e., "normal"--metal, the latter serving to pin the flux lines in the composite. While generally similar to the composites of the present invention, the IGC composites incorporates as a normal metal one that "will not diffuse, or will diffuse only nominally" (volume 2, lines 47-48) into the superconducting alloy. By contrast, the present invention requires that a significant fraction of the normal metal diffuses into the adjacent layers in the composite. Only in this way are the desired superconducting layers created.